A mineralogical reason why all exoplanets cannot be equally oxidising
Monthly notices of the Royal Astronomical Society (2023) stad2486
Abstract:
From core to atmosphere, the oxidation states of elements in a planet shape its character. Oxygen fugacity (fO2) is one parameter indicating these likely oxidation states. The ongoing search for atmospheres on rocky exoplanets benefits from understanding the plausible variety of their compositions, which depends strongly on their oxidation states—and if derived from interior outgassing, on the fO2 at the top of their silicate mantles. This fO2 must vary across compositionally-diverse exoplanets, but for a given planet its value is unconstrained insofar as it depends on how iron (the dominant multivalent element) is partitioned between its 2+ and 3+ oxidation states. Here we focus on another factor influencing how oxidising a mantle is—a factor modulating fO2 even at fixed Fe3+/Fe2+—the planet’s mineralogy. Only certain minerals (e.g., pyroxenes) incorporate Fe3+. Having such minerals in smaller mantle proportions concentrates Fe3+, increasing fO2. Mineral proportions change within planets according to pressure, and between planets according to bulk composition. Constrained by observed host star refractory abundances, we calculate a minimum fO2 variability across exoplanet mantles, of at least two orders of magnitude, due to mineralogy alone. This variability is enough to alter by a hundredfold the mixing ratio of SO2 directly outgassed from these mantles. We further predict that planets orbiting high-Mg/Si stars are more likely to outgas detectable amounts of SO2 and H2O; and for low-Mg/Si stars, detectable CH4, all else equal. Even absent predictions of Fe3+ budgets, general insights can be obtained into how oxidising an exoplanet’s mantle is.
Mantle mineralogy limits to rocky planet water inventories
Monthly notices of the Royal Astronomical Society 521:2 (2023) 2535-2552
Abstract:
Nominally anhydrous minerals in rocky planet mantles can sequester oceans of water as a whole, giving a constraint on bulk water inventories. Here we predict mantle water capacities from the thermodynamically-limited solubility of water in their constituent minerals. We report the variability of mantle water capacity due to (i) host star refractory element abundances that set mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass. We find that planets large enough to stabilise perovskite almost unfailingly have a dry lower mantle, topped by a high-water-capacity transition zone which may act as a bottleneck for water transport within the planet's interior. Because the pressure of the ringwoodite-perovskite phase boundary defining the lower mantle is roughly insensitive to planet mass, the relative contribution of the upper mantle reservoir will diminish with increasing planet mass. Large rocky planets therefore have disproportionately small mantle water capacities. In practice, our results would represent initial water concentration profiles in planetary mantles where their primordial magma oceans are water-saturated. We suggest that a considerable proportion of massive rocky planets' accreted water budgets would form surface oceans or atmospheric water vapour immediately after magma ocean solidification, possibly diminishing the likelihood of these planets hosting land. This work is a step towards understanding planetary deep water cycling, thermal evolution as mediated by rheology and melting, and the frequency of waterworlds.
Blue marble, stagnant lid: Could dynamic topography avert a waterworld?
The Planetary Science Journal 3:3 (2022)
Abstract:
Topography on a wet rocky exoplanet could raise land above its sea level. Although land elevation is the product of many complex processes, the large-scale topographic features on any geodynamically-active planet are the expression of the convecting mantle beneath the surface. This so-called "dynamic topography" exists regardless of a planet's tectonic regime or volcanism; its amplitude, with a few assumptions, can be estimated via numerical simulations of convection as a function of the mantle Rayleigh number. We develop new scaling relationships for dynamic topography on stagnant lid planets using 2D convection models with temperature-dependent viscosity. These scalings are applied to 1D thermal history models to explore how dynamic topography varies with exoplanetary observables over a wide parameter space. Dynamic topography amplitudes are converted to an ocean basin capacity, the minimum water volume required to flood the entire surface. Basin capacity increases less steeply with planet mass than does the amount of water itself, assuming a water inventory that is a constant planetary mass fraction. We find that dynamically-supported topography alone could be sufficient to maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately 10−4 times their mass, in the most favourable thermal states. By considering only dynamic topography, which has ~1-km amplitudes on Earth, these results represent a lower limit to the true ocean basin capacity. Our work indicates that deterministic geophysical modelling could inform the variability of land propensity on low-mass planets.
Low volcanic outgassing rates for a stagnant lid Archean Earth with graphite-saturated magmas
Physics of the Earth and Planetary Interiors 320 (2021) 106788
Abstract:
Volcanic gases supplied a large part of Earth's early atmosphere, but constraints on the value of this flux are scarce. Here we model how C-O-H outgassing could have evolved through the late Hadean and early Archean, under the conditions that global plate tectonics had not yet initiated, all outgassing was subaerial, and graphite was the stable carbon phase in the melt source regions. The model fully couples numerical mantle convection, partitioning of volatiles into the melt, and chemical speciation in the gas phase. The mantle oxidation state (which may not have reached late Archean values in the Hadean) is the dominant control on individual species' outgassing rates because it affects both the carbon content of basaltic magmas and the speciation of degassed volatiles. Volcanic gas from mantles more reduced than the iron-wüstite mineral redox buffer would contain virtually no CO2 because (i) carbonate ions dissolve in magmas only in very limited amounts, and (ii) almost all degassed carbon takes the form of CO instead of CO2. For oxidised mantles near the quartz-fayalite-magnetite buffer, we predict median CO2 outgassing rates of less than approximately 5 Tmol yr−1, still lower than the outgassing rates used in many Archean climate studies. Relatively weak outgassing is due in part to the redox-limited CO2 contents of graphite-saturated melts, and also to a stagnant lid regime's inefficient replenishment of upper mantle volatiles. Our results point to certain chemical and geodynamic prerequisites for sustaining a clement climate with a volcanic greenhouse under the Faint Young Sun.
Mantle redox state drives outgassing chemistry and atmospheric composition of rocky planets
Scientific Reports 10 (2020)
Abstract:
Volcanic degassing of planetary interiors has important implications for their corresponding atmospheres. The oxidation state of rocky interiors affects the volatile partitioning during mantle melting and subsequent volatile speciation near the surface. Here we show that the mantle redox state is central to the chemical composition of atmospheres while factors such as planetary mass, thermal state, and age mainly affect the degassing rate. We further demonstrate that mantle oxygen fugacity has an effect on atmospheric thickness and that volcanic degassing is most efficient for planets between 2 and 4 Earth masses. We show that outgassing of reduced systems is dominated by strongly reduced gases such as H2, with only smaller fractions of moderately reduced/oxidised gases (CO, H2O). Overall, a reducing scenario leads to a lower atmospheric pressure at the surface and to a larger atmospheric thickness compared to an oxidised system. Atmosphere predictions based on interior redox scenarios can be compared to observations of atmospheres of rocky exoplanets, potentially broadening our knowledge on the diversity of exoplanetary redox states.